This invention relates generally to electrophotography and in particular to a method and apparatus for measuring the effective concentration of electrostatic toner powder mixed with a carrier medium. More specifically, the invention relates to method and apparatus, whereby the ability of a xerographic carrier developing material to develop a xerographically formed electrostatic latent image with an optimum image density can be determined.
More particularly, the invention relates to a method and apparatus for determining the concentration of toner particles in a particulate developer mix comprising a triboelectric mixture of electroscopic toner particles and carrier particles. The apparatus of the present invention is particularly useful in electrostatic printing systems wherein electrostatic charge patterns are rendered visible by the application thereto of a particulate triboelectric developer mix of pigmented toner particles and carrier particles.
In order to develop an electrostatic latent image, it is required to dust the image with a developer powder, whereby the powder particles are selectively attracted to the charged areas to form a visible powder particle image of the electrostatic latent image. Development of the image is effected with developers which comprise in general, a mixture of a suitable pigmented or dyed electrostatic powder, hereinafter referred to as "toner" and a granulated carrier material termed "carrier" which by means of cascading over the image functions to carry and to generate triboelectric charges on the toner. These materials are conventional and well known.
The general process of development to which this invention relates is termed "carrier development" but may also be used with other developing systems such as magnetic brush development systems. In general, in carrier development the toner composition is loosely coated on the carrier surface to which it remains loosely affixed by reason of electrostatic attraction thereto. The type of carrier development most widely used commercially is called "granular" or "cascade" carrier development. This system is more fully described in U.S. Pat. No. 2,618,551 to L.E. Walkup and U.S. Pat. No. 2,638,416 to Walkup and Wise. In this process the electroscopic toner is desirable mixed with a granular carrier, either electrically conducting or insulating, magnetic or non-magnetic, provided that the particles of granular material when brought in close contact with the toner particles acquire a charge having an opposite polarity to that of the granular carrier particles and adhere to and surround the granular carrier particles.
In this development process, only the toner particles are consumed and the carrier particles remain and are constantly reused. Consequently, it is necessary to replenish the developer mix with additional toner particles periodically or continuously to maintain the concentration of toner particles in the developer mix within predetermined limits and to insure developed prints of proper density. If, for example, the concentration of toner particles in the developer mix is less than an optimum concentration, the density of the developer charge pattern is too light. On the other hand, if the concentration of the toner particles in the developer mix is too high, the excessive toner particles tend to adhere to the non-image areas of the print, providing prints with a "dirty," grayish background.
It has been proposed to monitor the concentration of toner particles in the developer mix by photoelectric methods, such as by measuring the light reflected from samples of developer mix, or by measuring the light from developed electrostatic charge patterns, but such apparatus is relatively complex, requires delicate adjustments, and is affected by external conditions, such as dust, the color of the pigment toner, and the color of the recording element involved.
Other measuring systems take into consideration variations in magnetic permeability, dielectric permittivity, electrical conductivity or combinations of these to indicate variations in the ratio of carrier to toner. For example, as described in U.S. Pat. No. 3,802,381 an externally applied electric or magnetic field is established in the area of a quantity of developer mix consisting of a toner and a carrier and a measurement of one or more of the aforementioned parameters is employed to indicate the need for a greater or lesser percentage of toner or carrier in the developer. The apparatus for indicating the carrier/toner ratio includes provisions for establishing an electric or magnetic field in a quantity of mix and provisions for providing an output signal representative of one or more of these parameters exhibited by the developer mix. Thus, this toner concentration detection technique utilizes the toner carrier mixture as a passive circuit element.
In addition, some of the known toner concentration control systems have the deficiency that when the developer tribo is very low due to developer material fatigue (degradation) for example, or after a prolonged period during which the copier was not used, as over a weekend; detect this as low toner concentration and further damage the quality of prints by increasing an already normal toner concentration above the acceptable level.